Inoculation alloy against micro-shrinkage cracking for treating cast iron castings

ABSTRACT

Inoculation alloys for treating cast iron castings containing, by weight, 0.005% to 3% of an element selected from the group consisting of bismuth, lead and antimony, 0.3 to 10% of metals of the rare-earth group and optionally aluminum up to 5%, and calcium up to 1.5%, the remainder being ferro-silicon, lanthanum constituting more than 90% of the rare-earth metals contained in the composition. The inventive alloys enable efficient inoculation of cast iron and avoid occurrence of micro-shrinkage cracking in the cast parts. The alloys are conditioned in the form of slugs or powder.

DOMAIN OF THE INVENTION

The invention relates to the treatment of liquid cast iron formanufacturing parts for which it is required to obtain a structure withno iron carbides and no micro-shrinkage cavities.

STATE OF THE ART

Cast iron is a well-known iron-carbon-silicon alloy widely used formanufacturing of mechanical parts. It is known that to obtain goodmechanical properties of these parts, it is important to eventuallyobtain an iron+graphite structure, minimising the formation of Fe₃C typeiron carbides that make the alloy hard and brittle.

It may be desirable for the graphite formed to be spheroidal, vermiformor lamellar, but the essential prior condition to be satisfied is toavoid the formation of iron carbide. To achieve this, the liquid castiron is subjected to an inoculation treatment before casting, thatfacilitates the appearance of graphite rather than iron carbide duringcooling.

Therefore the inoculation treatment is very important. It is well knownthat the efficiency of inoculation on liquid cast iron reduces withtime, regardless of the inoculants used, and the efficiency hasgenerally dropped by 50% after about 10 minutes; an expert in thesubject refers to this phenomenon as the “fading effect”. To achievemaximum efficiency, progressive inoculation is usually performedconsisting of making several additions of inoculants at different stagesof production of the cast iron. Thus, liquid cast iron is frequentlyinoculated, firstly in the ladle using an inoculating alloy, for examplemade of grains with a size of between 2 and 10 mm or between 0.4 and 2mm, and secondly “by jet”, in other words when the ladle is being pouredusing an inoculating alloy with grain sizes of between 0.2 and 0.7 mm,and finally “in the mould”, in fact in mould supply ducts, by usinginserts composed of an inoculating material along the path followed bythe liquid cast iron.

These inserts with a defined shape are called slugs. There are two typesof slugs:

-   -   “cast” slugs obtained by casting a molten inoculant,    -   agglomerated slugs obtained from a compacted powder usually with        a very small quantity of binder or possibly without any binder        at all.

An expert in the subject considers that cast slugs have the bestquality; however, agglomerated slugs are often preferred for costreasons. Since the casting time of a part is very short, the dissolutionkinetics of the slugs must be very fast.

Moreover, an expert in the subject very frequently observes voids inparts with dimensions measured in millimetres or micrometers, referredto as micro-shrinkage cavities. These defects make parts more brittle;moreover, if the parts have to be machined afterwards, for example tostraighten a surface, the presence of such a defect on the surface willinevitably make it necessary to scrap the defective parts.

One known means of preventing the appearance of micro-shrinkage cavitiesin cast iron parts is to add lanthanum into the liquid iron. This metalin the lanthanum groups has the property of reducing the viscosity ofthe iron, not only of liquid iron just before the beginning of itssolidification, but also during solidification, in other words thesolid+liquid mix. Everything happens as if adding lanthanum makes thecast iron become thixotropic. Thus an expert in the subject, if hedesigns the moulds correctly, can collect all the shrinkage cavities inthe feeder head and thus obtain sound parts.

Thus, nodulising agents containing lanthanum have been successfullymarketed, and are reserved for use in nodular cast irons called SG castirons, and FeSi type inoculants with 45% Si and 2% La have also beenmarketed, that can be used equally well for SG cast irons and forlamellar graphite cast irons, called LG cast irons.

The purpose of the invention is to provide inoculating alloys that canbe used to treat liquid cast iron enabling efficient inoculation,particularly during treatment “in the mould”, preventing the formationof micro-pores in parts obtained by casting.

OBJECT OF THE INVENTION

The object of the invention is inoculating alloys that will be used forthe treatment of cast iron containing (by weight) 0.005 to 3% of anelement in the bismuth, lead and antimony group, 0.3 to 10% of metals inthe group consisting of rare earths and possibly up to 5% of aluminiumand up to 1.5% of calcium, the remainder being ferro-silicon, lanthanumaccounting for more than 90% of the rare earths metals used in itscomposition.

The alloy preferably contains between 0.2 and 1.5% of bismuth, andpreferably between 0.7 and 1.3%. The content of lanthanum isadvantageously between 0.3 and 8%, and preferably between 0.5 and 3%.The presence of at least 0.8% of aluminium is advantageous, and itscontent is preferably between 1 and 3.5%.

The alloy according to the invention may be conditioned in the form of apowder or a mix of alloy powders with different compositions, or in theform of slugs moulded from the molten alloy, or agglomerated from apowder or a mix of powders. This powder preferably has a grain sizesmaller than 1 mm, with a size grading fraction between 50 and 250 μmaccounting for more than 35% of the total weight, and a fraction smallerthan 50 μm representing less than 25% of the total.

DESCRIPTION OF THE INVENTION

Since an inoculant is inherently intended to obtain cast iron withcarbon present in the form of graphite, the applicant thought that itwould be desirable to develop an inoculant with anti micro-shrinkagecavity properties.

Thus, the first step was to envisage inoculating alloys based on 75%FeSi with an added anti micro-shrinkage cavity element that could belanthanum or germanium. Required contents of germanium vary from 0.3 to6%. Required contents of lanthanum vary from 0.3 to 8%, and preferablyfrom 0.5 to 5%.

But more attractive solutions appeared by imagining inoculating alloysin which the same element could fulfil several functions: thus, it wasfound to be particularly attractive to start from an alloy like thatdescribed in U.S. Pat. No. 4,432,793 (Nobel-Bozel) based onferro-silicon and containing up to 3% of bismuth, lead or antimony, andup to 3% of rare earths, adding an anti-micro-porosity element to itsuch as lanthanum, and contracting the formula obtained by optimisingthe total amount of lanthanum and other rare earths in the Fe-Si-Bi-Laalloy.

The applicant started by checking that these new anti-micro-porosityalloys conditioned in normal size gradings, namely between 2 and 7 mm,or between 0.4 and 2 mm for treatment in ladles, and between 0.4 and 0.7mm for treatment in jets, had good properties as inoculants. The nextstep was to envisage the preparation of inoculating slugs with thesesame alloys. The result in terms of reduction of the micro-porosity wasconfirmed by the added bismuth in the final cast iron.

Thus, very good results were obtained with cast slugs composed of anFeSi type alloy containing:

-   -   from 60 to 80%, and preferably from 72 to 78% of silicon,    -   from 0.3 to 8%, and preferably from 0.5 to 5% of lanthanum,    -   from 0.2 to 1.5%, and preferably from 0.7 to 1.3% of bismuth,    -   from 0.8 to 5% and preferably from 1% to 3.5% of aluminium.

EXAMPLES

The examples described below were made by melting a cast iron charge inan induction furnace and treated using the Tundish Cover process using anormal FeSiMg type inoculating alloy with 5% of Mg and 1% of Ca notcontaining rare earths, using the dose of 20 kg for 1600 kg of castiron. The analysis of the liquid cast iron was as follows:

-   -   C=3.7%, Si=2.6%, Mn=0.07%, P=0.03%, S=0.003%, Mg=0.038%.

The performance in terms of macro-porosity and micro-porosity wasevaluated using the “V” test pieces casting test.

In this test, the test piece is composed of a 110 mm high “V” with anangle at the vertex equal to 40° C., the width of the branches of the“V” being 20 mm and the thickness of the part being 20 mm. This geometryresults in a width of 80 mm at the vertex of the “V”, a unit volume of69 cm³, and a unit mass of 480 g to 500 g depending on the quality ofthe cast iron. Pores in this type of part appear selectively in there-entrant part of the “V”.

To evaluate the test result, the part is cut at mid-thickness, and thesection is examined by optical microscopy to evaluate the pore surface;the result is expressed as a surface area of pores as a fraction of thesurface area of the section.

Example 1

A treated cast iron ladle originating from the preliminary operation wasinoculated in the ladle using a powder inoculating alloy with a sizegrading between 2 and 10 mm, with a “Foundry Grade” composition, theremainder being mainly Fe, used at a dose of 200 g per tonne of castiron.

This cast iron was used to cast V parts with geometry identical to thatdefined in the control test, arranged in clusters in a 36-part sandmould supplied by an inlet duct in which there is a filter composed of arefractory foam.

The parts obtained were examined by optical microscopy on a polishedsection to determine the metal structure as a function of the porositydepth and level.

The density of graphite modules at the heart of the branches wasmeasured at 120/mm².

The average porosity of the parts was evaluated at 2.4%.

Example 2

A second treated cast iron ladle from the preliminary operation wasinoculated in the ladle using an inoculating alloy with a size gradingof between 2 and 10 mm of composition:

-   -   Si=75.4%, Al=0.94%, Ca=0.86%, La=2.2%, Bi=0.92%, remainder        mainly Fe, used at a dose of 200 g per tonne of cast iron.

This iron was used to cast V parts with geometry identical to thatdefined in the control test, arranged in clusters in a 36-part sandmould supplied by an inlet duct in which there is a filter composed of arefractory foam.

The parts obtained were examined by optical microscopy on a polishedsection to determine the metal structure as a function of the porositydepth and level. The density of graphite modules at the heart of thebranches was measured at 360/mm².

The average porosity of the parts was evaluated at 0.3%.

Example 3

A third treated cast iron ladle originating from the preliminaryoperation was used to cast V parts with geometry identical to thatdefined in the control test, arranged in clusters in a 36-part sandmould supplied by an inlet duct in which 25 g slug is located composedof an inoculating alloy for treatment in the mould, with composition:

-   -   Si=73.6%, Al=3.92%, Ca=0.78%, La=2.1%, Bi=0.97%, remainder        mainly Fe. The parts obtained were examined by optical        microscopy on a polished section to determine the metal        structure as a function of the porosity depth and level. The        density of graphite modules at the heart of the branches was        measured at 320/mm².

The average porosity of the parts was evaluated at 0.2%.

1. Inoculating alloy for cast iron containing (by weight) 0.005 to 3% ofan element in the bismuth, lead and antimony group, 0.3 to 10% of metalsin the group consisting of rare earths and possibly up to 5% ofaluminium and up to 1.5% of calcium, the remainder being ferro-silicon,characterised in that lanthanum accounts for more than 90% of the rareearth metals used in its composition.
 2. Alloy according to claim 1,characterised in that it contains from 0.3 to 8% of lanthanum and from0.2 to 1.5% of bismuth.
 3. Alloy according to claim 1, characterised inthat it contains between 0.7 and 1.3% of bismuth.
 4. Alloy according toclaim 1, characterised in that it contains between 0.5 and 5% oflanthanum.
 5. Alloy according to claim 1, characterised in that itcontains between 0.8 and 5% of aluminium.
 6. Alloy according to claim 5,characterised in that it contains between 1 and 3.5% of aluminium. 7.Alloy according to claim 1, characterised in that it is conditioned inthe form of a powder.
 8. Alloy according to claim 1, characterised inthat it is conditioned in the form of slugs for treatment “in themould”.
 9. Alloy according to claim 8, characterised in that the slug isobtained by moulding from molten alloy.
 10. Alloy according to claim 8,characterised in that the slug is obtained by agglomeration of a powder.11. Alloy according to claim 10, characterised in that the powder grainsize is smaller than 1 mm, with the size grading fraction between 50 and250 μm accounting for more than 35% of the total weight, and thefraction smaller than 50 μm representing less than 25%.
 12. Alloyaccording to claim 10, characterised in that the average composition ofthe alloy is obtained by a mix of alloy powders with differentcompositions